Precision machinery such as printers is now manufactured with a lesser total weight by using gears made from synthetic resin. For the purpose of assuring sufficient rigidity and a higher precision thereof, such gears have their web formed in disc form, the disc having thereon reinforcing ribs.
With reference to FIG. 1 showing a spur gear applicable in this sort of precision machinery, the front view thereof is shown in FIG. 1(A), the cross-sectional view thereof taken on line A--A of FIG. 1(A) is shown in FIG. 1(B) and the rear view thereof is shown in FIG. 1(C).
The spur wheel 1 formed by injection molding of synthetic resin comprises a hub 2 having a length larger than face width b, and a web 3 of a disc form formed around the hub 2 and having a thickness smaller than the face width b. A circular rim 4 is formed around the web 3, and a predetermined number of teeth 5 are formed around the rim 4, wherein one end surface of the hub 2 is even with one end surface of the rim 4 while the other end surface of the hub 2 protrudes from the other end surface of the rim 4. Further, the web 3 is formed with a predetermined thickness around a virtual surface dividing the face width b into substantially equal parts. Accordingly, the spur gear 1 has a void space of less thickness between hub 2 and rim 4 on the both sides of the web 3, thereby to decrease the quantity of synthetic resin used and thus the total weight of gear for the quantity. Further, the web 3 is formed in disc shape, thereby to prevent a non-uniform deformation in the manufacturing process thereof so as to increase the accuracy of gear.
In the void space, spur gear 1 has a plurality of radial ribs 8A and 8B protruding from the hub 2 in order to assure its rigidity. More specifically, these ribs 8A and 8B are formed on both sides of the web 3 thereby to form a plate of total thickness of the face width b together with the web 3.
Further, the ribs 8A and 8B are arranged in axial symmetry with respect to a virtual line binding each gate 9 and the rotation axis of the hub 2 in consideration of resin fluidity, thereby to mold respective ribs with substantially same injection pressure. As described above, even number of ribs 8A and 8B are formed in general in this kind of spur gear 1, thereby to increase the rigidity of the web 3 having disc form for assuring the size accuracy of spur gear 1 thus obtained.
In this sort of spur gear 1, if its size accuracy can be increased further while maintaining its rigidity, an improvement of quality of printed characters can be expected from a printer for example using the spur gear 1.